Conventional electronic control units for vehicles are provided with functions of monitoring the operation of a microcomputer mounted therein as disclosed in JP 1998-507805A (U.S. Pat. No. 5,880,568) and JP 1999-505587A (U.S. Pat. No. 6,125,322). These electronic control units include a monitoring integrated circuit (IC) in addition to the microcomputer so that queries are transmitted from the monitor IC to the microcomputer. The microcomputer runs a test corresponding to a query from the monitor IC and transmits the result of the test to the monitor IC as a reply. The monitor IC compares the reply from the microcomputer with an expected value within the monitor IC to monitor the operation of the microcomputer.
The monitor IC is constructed to select a query to the microcomputer (that is, a test to be run in the microcomputer) and compare a reply from the microcomputer for the query with an expected value. As a result, the monitor IC needs to have a function to process replies from the microcomputer and a function to select a query, with the result that the construction of the monitor IC is complicated.
Furthermore, since the monitor IC separate from the microcomputer selects a query, it is difficult to achieve an attempt to run a test suitable for a current control mode in the microcomputer, for example. Since the monitor IC cannot recognize control modes of the microcomputer, it is impossible to select a query so that a test suitable for a current control mode is run.